<!doctype html>
<meta charset="utf-8">
<title>Test reference</title>
<style>
label[for="two"] {
  background: green;
}
</style>
<form>
  <input type="radio" name="a" id="one" value="1">
  <label for="one">One</label>

  <input type="radio" name="a" id="two" value="2" checked>
  <label for="two">Two</label>

  <input type="radio" name="a" id="three" value="3">
  <label for="three">Three</label>

  <input type="radio" name="a" id="four" value="4">
  <label for="four">Four</label>
</form>
